The widespread use of glass in the preparation and storage of foods, pharmaceuticals, beverages, and chemicals has occasioned extensive investigation into the factors influencing the resistance of glass surfaces to chemical attack, such resistance being referred to as "chemical durability". As a general rule, it has been found that glasses providing good durability to acid attack, and those providing good durability to alkaline attack, are mutually excludable. Low expansion borosilicate glasses having excellent acid durability are commercially available, but these glasses are quite prone to alkaline attack. Likewise, soda lime glasses having reasonably good alkaline durability, and special zirconia glasses having enhanced alkaline durability, are known, but the former are not acid resistant and the latter are difficult to melt and work.